How long does it take foam - styrofoam - polystyrene to decompose?

The decomposition times for various materials are often cited on green sites. Typically these are similar to:

• Banana Peel: 3-4 weeks
• Paper Bag: 1 month
• Cardboard: 2 months
• Wool Sock : 1 year
• Tinned Steel Can: 50 years

The following figures are misleading, because they typically include decomposition:

• Aluminum Can: 200-500 years (But if recycled, it can be reused within 6 weeks!)
• Disposable Diapers: 500 years (closer to 25-100 years)
• Plastic Bags : 20-1000 years (most will be crumbled or incinerated)
• Plastic Jug: 1 million years (polymers break down within 100 years)
• Glass : 1 million years (silica shards, like sand, are a stable molecule)
• Styrofoam: 1 million years (most will be powdered into grains within 100 years)

The problem is that the numbers never suggest the mechanism of decomposition. There are several - heat, biochemical, mechanical, irradiation. In the case of styrofoam, that material is biologically inert (microorganisms have a hard time eating it) so a long decomposition time is expected. However it is subject to degradation when exposed to sunlight (UV radiation), thermal energy (the bonds break when it gets hot enough), mechanical degradation (it erodes and wears away) and chemical attack (it burns). Under these conditions the decomposition time is much shorter.
 

A:

Exposure to UV from sunlight will degrade the styrene molecule. Exposure to ambient ozone will also degrade the styrene molecule.
In the sun, in a place with lots of ozone pollution, styrofoam falls apart in months. Built into refrigerators and ice chests, sealed away from ozone exposure with anti-oxidants built in, stryrofoam will last for (probably) centuries. In a landfill, styrofoam usually lasts about a decade, depending on moisture and biological history.